Gregory Gabadadze, Siqing Yu

In a recently proposed theory, the cosmological constant (CC) does not curve spacetime in our universe, but instead gets absorbed into another universe endowed with its own dynamical metric, nonlocally coupled to ours. Thus, one achieves a long standing goal of removing entirely any cosmological constant from our universe. Dark energy then cannot be due to a cosmological constant, but must be obtained via other mechanisms. Here we focus on the scenario in which dark energy is due to massive gravity and its extensions. We show how the metric of the other universe, that absorbs our CC, also gives rise to the fiducial metric known to be necessary for the diffeomorphism invariant formulation of massive gravity. This is achieved in a framework where the other universe is described by 5D AdS gravity, while our universe lives on its boundary and is endowed with dynamical massive gravity. A non-dynamical pullback of the bulk AdS metric acts as the fiducial metric for massive gravity on the boundary. This framework also removes a difficulty caused by the quantum strongly coupled behavior of massive gravity at the Lambda3 scale: in the present approach, the massive gravity action does not receive any loop-induced counterterms, despite being strongly coupled.

jbm120

http://bicepkeck.org/BK-VI_20151029.pdf

We present results from an analysis of all data taken by the BICEP2 & Keck Array CMB polarization experiments up to and including the 2014 observing season. This includes the first Keck Array observations at 95 GHz. The maps reach a depth of 50 nK deg in Stokes Q and U in the 150 GHz band and 127 nK deg in the 95 GHz band. We take auto- and cross-spectra between these maps and publicly available maps from WMAP and Planck at frequencies from 23 GHz to 353 GHz. An excess over lensed-ΛCDM is detected at modest significance in the 95×150 BB spectrum, and is consistent with the dust contribution expected from our previous work. No significant evidence for synchrotron emission is found in spectra such as 23×95, or for dust/sync correlation in spectra such as 23×353. We take the likelihood of all the spectra for a multi-component model including lensed-ΛCDM, dust, synchrotron and a possible contribution from inflationary gravitational waves (as parametrized by the tensor-to-scalar ratio r), using priors on the frequency spectral behaviors of dust and synchrotron emission from previous analyses of WMAP and Planck data in other regions of the sky. This analysis yields an upper limit r0.05 < 0.09 at 95% confidence, which is robust to variations explored in analysis and priors. Combining these B-mode results with the (more model-dependent) constraints from Planck analysis of CMB temperature and other evidence yields a combined limit r0.05 < 0.07 at 95% confidence. These are the strongest constraints to date on inflationary gravitational waves

Jeppe Dakin

CO$N$CEPT (COsmological $N$-body CodE in PyThon) is a free and open-source code for cosmological $N$-body simulations on massively parallel computers with distributed memory. Collisionless dark matter is the only implemented particle species. Gravity can be computed using the PP, PM or the P$^{3}$M algorithm. The goal of CO$N$CEPT is to make it pleasant to work with cosmological $N$-body simulations - for the cosmologist as well as for the source code developer. This is the user guide. The source code and additional documentation can be found at https://github.com/jmd-dk/concept/

Leonardo Leitao, Ariel Megevand

In cosmological first-order phase transitions, gravitational waves are generated by the collisions of bubble walls and by the bulk motions caused in the fluid. A sizeable signal may result from fast-moving walls. In this work we study the hydrodynamics associated to the fastest propagation modes, namely, ultra-relativistic detonations and runaway solutions. We compute the energy injected by the phase transition into the fluid and the energy which accumulates in the bubble walls. We provide analytic approximations and fits as functions of the net force acting on the wall, which can be readily evaluated for specific models. We also study the back-reaction of hydrodynamics on the wall motion, and we discuss on the extrapolation of the friction force away from the ultra-relativistic limit.

Dominik J. Schwarz, Craig J. Copi, Dragan Huterer, Glenn D. Starkman

Several unexpected features have been observed in the microwave sky at large angular scales, both by WMAP an by Planck. Among those features is a lack of both variance and correlation on the largest angular scales, alignment of the lowest multipole moments with one another and with the motion and geometry of the Solar System, a hemispherical power asymmetry or dipolar power modulation, a preference for odd parity modes and an unexpectedly large cold spot in the Southern hemisphere. The individual p-values of the significance of these features are in the per mille to per cent level, when compared to the expectations of the best-fit inflationary $\Lambda$CDM model. Some pairs of those features are demonstrably uncorrelated, increasing their combined statistical significance and indicating a significant detection of CMB features at angular scales larger than a few degrees on top of the standard model. Despite numerous detailed investigations, we still lack a clear understanding of these large-scale features, which seem to imply a violation of statistical isotropy and scale invariance of inflationary perturbations. In this contribution we present a critical analysis of our current understanding and discuss several ideas of how to make further progress.

C. Armendariz-Picon

We point out that in theories in which the gravitational couplings depend on the inflaton, the standard relation between the primordial tensor amplitude and the scale of inflation is lost. This mostly happens because the Planck mass that determines the tensor amplitude does not need to agree with the value of the Planck mass that we infer from the matter gravitational interactions. We also briefly speculate that the same mechanism may shed some light on the cosmological constant problem.

Justin Vandenbroucke, Silvia Bravo, Peter Karn, Matthew Meehan, Matthew Plewa, Tyler Ruggles, David Schultz, Jeffrey Peacock, Ariel Levi Simons

In 2014 the number of active cell phones worldwide for the first time surpassed the number of humans. Cell phone camera quality and onboard processing power (both CPU and GPU) continue to improve rapidly. In addition to their primary purpose of detecting photons, camera image sensors on cell phones and other ubiquitous devices such as tablets, laptops and digital cameras can detect ionizing radiation produced by cosmic rays and radioactive decays. While cosmic rays have long been understood and characterized as a nuisance in astronomical cameras, they can also be identified as a signal in idle camera image sensors. We present the Distributed Electronic Cosmic-ray Observatory (DECO), a platform for outreach and education as well as for citizen science. Consisting of an app and associated database and web site, DECO harnesses the power of distributed camera image sensors for cosmic-ray detection.

Alessio Notari, Miguel Quartin

We show that the Cosmic Microwave Background can be used to measure our peculiar velocity in a novel way, by looking at Doppler-induced distortions of the intensity blackbody spectrum which couple different multipoles. The frequency dependence of such a signal is called y-type, and is degenerate with the thermal SZ (tSZ) effect. Interestingly, like the kinetic Doppler quadrupole, its measurement is not limited by cosmic variance of the temperature spectrum; instead it only depends on experimental noise and on the small contamination due to the tSZ effect. Already with Planck this method yields a signal-to-noise ratio of about 9, and future experiments can increase this to somewhere around 15-40, and in principle even further if tSZ effect can be subtracted using data from clusters. Such a signal is present at all multipoles, but mostly in ell <~ 400, providing thus an independent way to measure our velocity that might also clarify the mixing between Doppler and a possible anomalous intrinsic dipolar modulation of the CMB spectrum, which seems to be present in temperature data at large scales.

B. King, T. Heinzl

When exposed to intense electromagnetic fields, the quantum vacuum is expected to exhibit properties of a polarisable medium akin to a weakly nonlinear dielectric material. Various schemes have been proposed to measure such vacuum polarisation effects using a combination of high power lasers. Motivated by several planned experiments, we provide an overview of experimental signatures that have been suggested to confirm this prediction of quantum electrodynamics of real photon-photon scattering.

Beste Korutlu

The direct coupling between the Higgs field and the spacetime curvature, if finely tuned, is known to stabilize the Higgs boson mass. The fine-tuning is soft because the Standard Model (SM) parameters are subject to no fine-tuning thanks to their independence from the Higgs-curvature coupling. This soft fine-tuning leaves behind a large vacuum energy $\propto \Lambda_{\rm UV}^4$ which inflates the Universe with a Hubble rate $\propto \Lambda_{\rm UV}$, $\Lambda_{\rm UV}$ being the SM ultraviolet boundary. This means that the tensor-to-scalar ratio inferred from cosmic microwave background polarization measurements by BICEP2, Planck and others lead to the determination of $\Lambda_{\rm UV}$. The exit from the inflationary phase, as usual, is accomplished via decays of the vacuum energy. Here we show that, identification of $\Lambda_{\rm UV}$ with the inflaton, as a sliding UV scale upon the SM, respects the soft fine-tuning constraint and does not disrupt the stability of the SM Higgs boson.